ABSTRACT

Dissolved oxygen is a key ground-water constituent, controlling both
the geochemistry and microbiology of an aquifer. Two methods were
developed to analyze isotopes of dissolved O2:
the traditional method, which uses dual inlet mass spectrometry for
analyzing CO2 previously converted
from O2 , and a newly developed
method, which uses continuous flow isotope-ratio mass spectrometry
for analyzing directly the isotopes of dissolved O2
. The major differences between the two methods are in
the sample size and sample handling in the field and laboratory. Isotope
analyses of dissolved O2 were used
to document the occurrence of microbial respiration near the boundary
of an anoxic ground-water plume consisting of treated wastewater at
the Massachusetts Military Reservation on Cape Cod. The upper boundary
of the plume was a mixing zone between contaminated, anoxic plume
water and uncontaminated, oxic local recharge water. Concentrations
of dissolved oxygen decreased downward near the top of the plume and
were inversely correlated with 18O.
Apparent oxygen isotope fractionation factors (e) were -1 to -10 per
mil (‰) in the absence of a dilution correction, whereas values as
low as -20 ‰ were obtained by adjusting for the effects of dilution.